1. Field of the Invention
The present invention generally relates to a brake fluid pressure control apparatus and, more particularly, to a brake fluid pressure apparatus which is suitable for controlling a pressure of brake fluid in a brake apparatus for a vehicle.
2. Description of the Related Art
Conventionally, as described in Japanese Laid-Open Patent Application No. 4-232154, for example, a brake fluid pressure apparatus is known which comprises a master cylinder for generating a fluid pressure in response to a depression force of a brake and an electronically controlled fluid pressure generating mechanism for generating a brake fluid pressure in response to a designated value. The above-mentioned conventional apparatus includes a stroke sensor for detecting an amount of stroke of a brake pedal and a depression force sensor for detecting a brake depression force exerted on the brake pedal.
When the stroke sensor and the depression force sensor (hereinafter, these sensors are referred to as an operational force sensor as a whole) function normally, a level of brake force which a driver intends to generate can be assumed based on detection values of these sensors. When the operational force sensors are normally operated, the above-mentioned conventional apparatus controls an electronically controlled fluid pressure generating mechanism based on the detected values of the sensors, and adjusts a wheel cylinder pressure P.sub.W/C by using the electronically controlled fluid pressure generating mechanism as a fluid pressure source.
In the above-mentioned conventional apparatus, if an abnormality occurs in the operational force sensor, it is difficult to generate an appropriate brake fluid pressure by the electronically controlled fluid pressure generating mechanism in response to the brake operational force. Thus, in the above-mentioned conventional apparatus, when an abnormality is detected in the operational force sensor, each wheel cylinder is connected to a master cylinder instead of the electronically controlled fluid pressure generating mechanism.
In the above-mentioned structure, when the operational force sensor functions normally, the wheel cylinder pressure P.sub.W/C can be adjusted to a fluid pressure of a predetermined power ratio relative to a brake operational force by providing a brake fluid pressure generated by the electronically controlled fluid pressure generating mechanism to the wheel cylinder. On the other hand, when an abnormality occurs in the operational force sensor, the wheel cylinder pressure can be adjusted to an appropriate pressure by providing the master cylinder pressure to the wheel cylinder.
In the above-mentioned conventional brake fluid pressure control apparatus, when the master cylinder pressure P.sub.M/C is provided to the wheel cylinder, it is difficult to obtain a large brake force as compared to that obtained when the brake fluid pressure generated by the electronically controlled brake fluid generating mechanism is provided to the wheel cylinder. In this regard, the above-mentioned conventional apparatus has a characteristic that a large brake force is difficult to be obtained when an abnormality occurs in the operational force sensor.